1. Field of the Invention
This invention generally relates to detoxification of wastewaters containing dissolved and/or suspended solids. Generally speaking, cold oxidant-containing fluid is directly heated by a hot recycling fluid, and mixed with a cold waste fluid which contains compounds to be oxidized. Solid and gaseous products may be separated in situ by crossflow filtration attachments. Sintered materials may be used as filter, gas separation, or catalyst elements. Thermal energy generated by the process may be recovered. A novel high temperature pumping means is also described.
2. Description of the Prior Art
The concept of wet oxidation of wastewater and sludge has been used since the 1960s. The need for complete and effective destruction of hazardous waste materials resulted in further investigation of wet oxidation at reaction conditions near the supercritical point of water (about or above 705.degree. F. (374.degree. C.) and about or above 3205 psia (221 bar)). It has been demonstrated in lab-scale tests that supercritical water oxidation is a viable waste treatment alternative to meet new environmental regulations. The growing interest and demand for wet oxidation requires that a practical process and apparatus be developed to apply wet oxidation to waste and wastewater treatment.
Current literature and practice have taught that the thermal energy carried in the reactor effluent may be used to heat the incoming feed through heat exchangers located either outside or inside of the reaction vessel. For example, one known process utilizes external heat exchangers with a subsurface (i.e. underground) vertical reaction vessel process. Another known process utilizes internal heat exchangers. Other similar subsurface concentric tube reactor processes have been described. A problem with the heat utilization systems presently used is that scale formation in heat exchangers and/or heat exchange zones is often fast and severe. One cause of the scale formation is that the wastewater or sludge in the feed is relatively slowly heated, allowing time for scale products to form. Cleaning of the scale in heat exchange units requires service time and additional costs. Scale formation may also cause localized corrosion.
Conventional practice for removing residue solids in wet oxidation reactor vessels has been to shut down the wet oxidation system, completely drain the reactor, and then physically remove the solids. One known method periodically removes solids from a wet oxidation reactor, with only a temporary suspension of feed to the plant. This method apparently works without draining the entire liquid portion of the reactor contents, and without having to cool the reactor contents. Other systems use a solid separation apparatus in a two zone pressure vessel wherein precipitates and other solids fall or are sprayed from a supercritical temperature superzone into a lower temperature subzone. The feed material may comprise various waste products which are subsequently oxidized in the superzone. The resultant brine or slurry at the lower temperature subzone of the vessel is removed via a pipe.
Crossflow filtration has been used for removal of heavy metals and suspended solids from battery wastewaters. Lab-scale and pilot plant studies have also treated municipal sewage effluent by crossflow filtration. These studies, however, have been conducted at relatively low temperatures and pressures, and little, if any, information on crossflow filtration under high pressure and high temperature conditions is known.
Relatively little is also known about the use of catalysts for supercritical wet oxidation processes. The difficulty and cost of operating at supercritical conditions has deterred supercritical catalyst studies.
In the past, oxidants have been introduced in wet oxidation processes by either (1) premixing the oxidant-containing fluid and the compound to be oxidized, or (2) injecting the compound to be oxidized or the oxidant-containing fluid at various locations in heat exchange or reaction zones. Both types of processes tend to produce scale and/or char in the reactor because of lack of oxidant available (at reaction conditions) to the compound to be oxidized. In process (1), the oxidant tends to degrade or react before reaction conditions are reached (either because of instability of the oxidant, or because of reaction between the oxidant and the compound to be oxidized). Thus the amount of oxidant available when the mixture reaches reaction conditions is reduced, causing scale and/or char formation due to insufficient oxidation of the compounds to be oxidized at reaction conditions. In process (2) insufficient mixing often allows some of the compounds to be oxidized to reach reaction conditions prior to contacting the oxidant, thus again causing scale and/or char formation due to insufficient oxidation of the compounds to be oxidized at reaction conditions.
In addition to the above problems, high temperature and pressure reactor system development has been hindered because there presently is no known equipment for pumping fluids at supercritical or near-supercritical conditions wherein the equipment will not be damaged by those conditions in a short amount of time.